The invention relates to a data signal correction circuit comprising an amplitude correction circuit, a level correction circuit and a signal path, coupled to an output of the level correction circuit, for obtaining a correction signal. The signal path comprises a sign-determining circuit. The amplitude correction circuit and the level correction circuit are controllable by the correction signal and the amplitude correction circuit is switchable by an output signal of a zero-crossing detection circuit coupled to the signal path.
IEEE Transactions on Consumer Electronics, Vol. CE 27, August 1981, pages 512-529 disclose a data signal correction circuit of the above type. Therein, the correction signal is obtained with the aid of a difference-determining circuit to which the output signal of the level correction circuit is applied, on one hand, directly and, on the other hand through the sign-determining circuit which is constituted by a double-sided limiter and followed by the amplitude correction circuit.
It is an object of the present invention to provide a data signal correction circuit with a higher degree of accuracy.
According to the present invention, in a data signal correction circuit of the type described above, the correction signal is obtained from an output of the sign-determining circuit, and the sign-determining circuit is switchable by the zero-crossing detection circuit between a three-level sign-changing function in a normal operating mode and a one-level sign-changing function in the mode in which no zero crossings are detected, and the amplitude correction circuit is switchable by the zero-crossing detection circuit to a non-controlled amplification differing from zero, in the operating mode in which no zero crossings are detected.
Applicants have found that a difference in the time delay of the two signal paths to the difference determining circuit may produce deviations in the corrected signal in the prior art correction circuit. By using the sign-determining circuit which, in the normal operating mode, has a three-level sign-changing function, the signal path for obtaining the correction signal may be of a signal construction, as a result of which said time delay differences are eliminated.
The present invention is further based on the recognition that a sign-determining circuit with a three-level sign-changing function has a function sgn (sgn y-y) for the correction signal, where y is the input signal of the sign-determining circuit. Such a correction signal function is suitable for use in control systems of a data signal correction circuit.
In the undesired condition, in which no zero crossings are detected, the circuit is blocked when there is an input signal, which indicates that the level of the output signal of the level correction circuit is incorrect. After the absence of zero crossings has been detected, the correction signal function is temporarily switched to -sgn y by which the level correction circuit can correct the level and whereby the blocking is eliminated.